Interaction of albumin and fusidic acid.

1. By combining the agar plate diffusion technique for determination of antibiotic activity and zone microelectrophoresis in agar gel, the activity of fusidic acid in individual serum proteins of blood and pus obtained from patients given sodium fusidate revealed albumin to be responsible for the protein binding of this antibiotic.2. Based on the assumption that only free fusidic acid is microbiologically active, the relationship between the concentration of albumin and the ratio of free to total fusidic acid was determined at four concentrations of free fusidic acid, using as test organisms four differently sensitive variants of a Staphylococcus aureus strain. At each concentration an increasing amount of albumin (0-40 mg/ml culture medium) decreased the activity of fusidic acid as determined in serial dilutions (IC50).3. The law of mass action expressed as Langmuir's adsorption isotherm was valid if a correction for the influence of albumin on the sensitivity of the strain of Staph. aureus was introduced. For other test organisms no correction is necessary. The constant in Langmuir's adsorption isotherm was K=78400+/-8200 l./mol and n=3.15 (95% confidence limits: 2.09-5.52).4. The mean blood concentration was 20.8 mug/ml and the mean pus concentration 17.2 mug/ml in nineteen sets of blood and pus samples. The ratio of pus to blood corresponds to the ratio of published values for the protein concentrations in serum and in inflammatory oedema.5. It is concluded that for albumin bound drugs the ;storage depot' of the organism also includes the fluid of the tissue spaces including the inflammatory oedema. As recent studies have revealed an extravascular albumin pool similar in size to the plasma pool, this ;storage depot' should not be neglected.